Commercial nuclear reactors used for generating electric power include a core composed of a plurality of fuel assemblies which generate heat used for electric power generation purposes. Each fuel assembly includes an array of fuel rods and control rod guide tubes held in spaced relationship with each other by grids of egg-crate configuration spaced along the fuel assembly length. The fuel rods may be approximately 0.5 inches in diameter and about 12 feet long, thus requiring a number of support grids along their length. Each grid includes a plurality of interwoven Inconel or Zircaloy straps which are vertically stacked to form multiple cells, with each cell having springs on two adjacent walls and projections, such as arches, on each of the other two adjacent opposing walls. The springs impose lateral forces on each fuel rod in the assembly, pressing the fuel rods into contact with the opposing arches. Although this fuel assembly design performs exceptionally well in a nuclear reactor, one disadvantage inherent in this design is that the inwardly projecting springs and arches cause fuel rod fretting wear at the contact points between the fuel rods and the support grid springs and arches.
Fuel rod fretting wear is an important design consideration for pressurized water reactor (PWR) steam generators. Fuel rod wear results from fluid-flow-induced fuel-rod vibration and from the existence of clearances, or gaps, between the fuel rods and the fuel rod supports. Such gaps are either initially present or form during reactor operation. If fuel rod vibration is excessive in duration and intensity, wear can result in unacceptable fuel rod wall material loss and fatigue cracking, leading to fuel rod failure.